Process and configuration for providing external upflow/internal downflow in a continuous digester

ABSTRACT

A continuous digester configuration and process for continuously cooking and digesting cellulosic material and producing cellulosic pulp is disclosed. The digester configuration comprises a plurality of jump-stage recirculations located along the length of a continuous digester vessel, wherein each of the jump-stage recirculations moves extracted liquor from one elevation of the digester vessel and re-introduces the liquor at a higher elevation in the vessel to maintain an internal liquor downflow substantially throughout the entire digester vessel. The process for continuously cooking and digesting cellulosic material comprises maintaining an internal concurrent liquor downflow substantially throughout the continuous digester vessel, wherein the internal liquor downflow does not exceed 400 gallons per ton of b.d. wood feed.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to and the benefit of U.S.patent application Ser. No. 60/240,659, filed Oct. 16, 2000, which ishereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

[0002] This invention relates generally to continuous digesters thatdigest cellulosic fibrous material and produce cellulosic pulp. Moreparticularly, this invention relates to an improved digesterconfiguration and process for use in and with a continuous digester. Theconfiguration and process of this invention produce a heretofore-unseeninternal con-current flow or downflow in the digester vessel.

BACKGROUND OF THE INVENTION

[0003] Continuous digesters for production of pulp were developed in the1940's and commercialized in the 1950's. Original continuous digesterdesigns were completely con-current (downflow of chips,chip-bound-liquor, and liquor). Digesters sold in the 1950's weredescribed as “cold blow” digesters in that they had a set ofcircumferential extraction screens followed by cold filtrate addition atthe very bottom of the vessel, just prior to pulp discharge. Theextraction screens extracted hot, spent cooking liquor, which was thenreplaced with an even larger volume of cold filtrate from a downstreamwashing process. Filtrate addition was through a circumferential headerand a central pipe downcomer whose discharge was at or around theelevation of the extraction screens. Thus, digesters from the late1950's practiced a simultaneous extraction/dilution sequence at the veryend of the process.

[0004] In practice, the “cold blow” digesters were limited in the amountof hot liquor that could be extracted from the screens. These screenswere also prone to plugging. As a result, the discharge temperaturecould not be effectively controlled (i.e., lowered). Also, the lower theextraction flow, the higher the amount of by-products in the dischargestream. This required additional washing requirements from downstreamunit operations.

[0005] In response the problems cited above, a high-heat in-digesterwashing system was developed (circa early 1960's) and continues to beused today. In this system, the bottom of the digester vessel isconverted to a continuous counter-current contacting system whereinchips and chip-bound-liquor move vertically down while the interstitialliquor between chips (i.e., free-liquor) moves vertically up. Again,wash filtrate from a downstream washing process is added in excess atthe very bottom of the vessel. This filtrate serves as a washing mediumwithin the high-heat wash zone. The high-heat wash zone also served as aheat recovery process in that the hot chip/liquor mixture transferredheat to the net upflowing, cooler filtrate. This upflowing filtrateeffectively captures system heat and returns it back into the vessel(verses allowing it to be lost through discharge). Thus, filtrate isheated while pulp is simultaneously cooled for discharge at temperaturesbelow 185° F. In order to affect an upflow, liquor is extracted from aset of circumferential extraction screens located well above the bottomof the vessel (i.e., heights that correspond to chip retention timeswhich range from 30 minutes to 4 hours).

[0006] In the early 1960's, the continuous cooking system could be usedto affect a counter-current cooking process wherein cooking chemicalsare brought into contact with the chips and bound liquor in acounter-current manner. This process demonstrated advantages vis-a-visbetter strength properties, better bleaching properties, and improvedcooking uniformity.

[0007] The counter-current process was characterized by having (a) splitwhite liquor (WL) additions, (b) counter-current cooking with a majorcharge of WL to the very bottom of the vessel, (c) internalrecirculation of liquors, and (d) black liquor impregnation. Theinternal recirculation involved taking a portion of the liquor drawnfrom the mid-level extraction screens and redirecting it to feed, orfront-end, of the process. As such, this represented a jump-stage ofupflowing liquor such that the impregnation zone of the vessel (i.e.,the top most zone) was an internal-downflow, external-upflow system.

[0008] The counter-current process was further augmented by having splitWL additions introduced at various levels (or zones) within thecounter-current section of the system. This represents the first attemptto profile alkali concentrations along the height of the digester so asto optimize pulp composition and thus pulp yield and pulp properties.The process contains a recirculation from one elevation of the digesterto a second, higher (or upstream) elevation.

[0009] During the 1960's and 1970's, continuous digesters that usedhigh-heat washing became the industry standard. This trend continues tothis date. Compared to alternative technologies, namely batch cookingprocesses and continuous process without counter-current washing, thesesystems offered superior pulp cleanliness, pulp properties, reliability,and energy efficiency. In spite of this, counter-current cookingtechnology did not take hold. This can be attributed to numerousfactors, perhaps the most important being that there were insufficienteconomic incentives to mitigate potential risks and development costs.Nonetheless, the continuous cooking systems of the era did not havereliable, sufficiently high wash zone upflows. Limitations on the amountof stable upflow that could be obtained make the use of counter-flowingsystems for the purposes of cooking unattractive. In general, therelative amount of upflow is limited by (a) excessive column compactionat the extraction screens, (b) unstable column dynamics in the blowdilution zone, and (c) excessive drag forces in the counter-currentzone. In the final analysis, for digesters built in the 1960's and1970's, the relative amounts of upflow and extraction flow were severelylimited by their inherent design and this led to practical operatingconstraints. These problems were made even worse by increased productionrates (and thus increased loading rates), and many commercial systemswere (and still are) operated at much higher rates than their originaldesign.

[0010] During the late 1970's and early 1980's, considerable progresswas made towards improving the system design so as to allow forrelatively higher, more stable upflows. Significant innovations includednew screen plate designs, the use of switching valves to minimize headerplugging, and optimization of the vessel geometry. These improvementsdecreased column compaction at the extraction and provided more upflow.At the same time, renewed interest in modified cooking technologiesevolved, indirectly, from economic and social pressures to decrease theenvironmental impact of pulping and bleaching practices. In the late1970's and early 1980's, researchers performed systematic studies toidentify optimal pulping conditions. From this, the general principlesof modified cooking evolved. These principles state that pulpingselectivity can be optimized by (a) maintaining a lower alkaliconcentration at the start of bulk delignification, (b) maintaining ahigher sulfidity during impregnation and the start of bulkdelignification, (c) maintaining lower overall ionic strength,particularly at the end of bulk and throughout residual delignification,and (d) minimizing lignin concentration (while increasing relativealkali concentration) at the end of bulk delignification and throughoutresidual delignification. It so happened that the counter-currentcooking methods described earlier achieved all of these objectives. Fromthe mid 1980's onward, modified continuous cooking methods (MCC) becamethe industry standard. The combination of better system design, a betterfundamental understanding of process optimization requirements, and morerelevant driving forces made this technology a commercial reality.

[0011] The “Lo-Solids®” based processes (Marcoccia US Pat. Nos.5,489,363, 5,536,366, 5,547,012, 5,620,562, 5,662,775) are unique inthat they describe means for achieving the benefits of modified cookingin systems with little or no wash zone upflow. This is of greatimportance since the vast majority of installed commercial units stillhave difficulty maintaining sufficiently high, stable upflows (i.e.,even those built after the 1980's, but particularly those built before).

[0012] During the 1960's and 1970's, digesters were built with multiplefiltrate additions points (i.e., in addition to the bottom, provisionswere made for filtrate addition to the feed and to the extraction screenelevation via the so-called “quench” circulation). The simultaneousextraction dilution sequences at the quench (and at the bottom of coldblow digesters) is fundamentally different than the simultaneousextraction/dilution sequence prescribed by Lo-Solids® cooking due to thefact that these operations do not purge and dilute throughout bulkdelignification as prescribed by Lo-Solids®. Similarly, in the 1970'sand 1980's, several mills modified their digester configurations so asto be able to extract at both the extraction screens and wash (or bottomzone screens). In some cases, filtrate was introduced at the quench andin others a jump stage recirculation from the wash screen elevation upto the extraction screen elevation was used. The Lo-Solids® process wasdetermined to be unique form these earlier processes (even though bulkdelignification was known to occur downstream of the extraction screens)owing to the fact that these earlier processes did not purge and diluteat the start of middle of bulk delignification. Furthermore,jump-staging via internal recirculation emulates a counter-currentprocess wherein dissolved organic material (DOM) is pushed upstream(versus the cross-flow Lo-Solids® process where DOM is purged anddiluted).

[0013] In view of the above, however, there is still a continuing needfor alternative continuous digester configurations and processes toimprove digester performance and efficiency.

SUMMARY OF THE INVENTION

[0014] The present invention relates to continuous digesterconfigurations and processes designed to optimize heat recovery,optimize washing efficiency, optimize operations stability, and obtainthe benefits of modified cooking in a continuous digester.

[0015] In accordance with the purposes of this invention, as embodiedand broadly described herein, this invention, in one aspect, relates toa digester for continuously cooking and digesting cellulosic materialand producing cellulosic pulp comprising a plurality of jump-stagerecirculations located along the length of a continuous digester vessel,wherein each of the jump-stage recirculations moves extracted liquorfrom a first elevation of the digester vessel and re-introduces theextracted liquor into the vessel at a second higher elevation tomaintain an internal liquor downflow substantially throughout the entiredigester vessel.

[0016] In another aspect, this invention relates to a digester forcontinuously cooking and digesting cellulosic material and producingcellulosic pulp comprising a jump-stage recirculation located along thelength of a continuous digester vessel, wherein the jump-stagerecirculation moves extracted liquor from a first elevation of thedigester vessel and re-introduces the liquor into the vessel at a secondhigher elevation to maintain an internal liquor downflow substantiallythroughout the entire vessel that does not exceed 400 gallons per ton ofb.d. wood feed.

[0017] In yet another aspect, this invention relates to a process forcontinuously cooking and digesting cellulosic material and producingcellulosic pulp in a continuous digester comprising (a) maintaining aninternal, con-current liquor downflow substantially throughout acontinuous digester vessel, wherein the internal liquor downflow doesnot exceed 400 gallons per ton of b.d. wood feed.

[0018] Additional advantages of the invention will be set forth in partin the figures and detailed description, which follow, and in part willbe obvious from the description, or may be learned by practice of theinvention. The advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe appended claims. It is to be understood that both the foregoinggeneral description and the following detailed description are exemplaryand explanatory of preferred embodiments of the invention, and are notrestrictive of the invention, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a schematic view of an exemplary continuous digestersystem according to one embodiment of the present invention, showninterconnected to other components of a mill for producing cellulosepulp.

[0020]FIG. 2 is a schematic view of one embodiment of a continuousdigester configuration of the present invention.

[0021]FIG. 3 is a schematic view of another embodiment of a continuousdigester configuration of the present invention.

[0022]FIG. 4 is a schematic view of one embodiment of jump-stagerecirculations of the present invention shown associated withcircumferential digester screens.

[0023]FIG. 5 is a schematic view of yet another embodiment of acontinuous digester configuration of the present invention.

[0024]FIG. 6 is a schematic view of a known continuous digester and itsinternal liquor flow.

[0025]FIG. 7 is a schematic view of two alternative liquor downflowconfigurations of the present invention.

[0026]FIG. 8 is a schematic view of a side-by-side comparison of a knownliquor flow configuration and a liquor flow configuration according tothe present invention.

[0027]FIG. 9 is a graph of the effect of downflow cooking according tothe present invention on digester discharge.

[0028]FIG. 10 is a graph of the effect of downflow cooking according tothe present invention on filtrate-cooler duty.

[0029]FIG. 11 is a graph of the effect of downflow cooking according tothe present invention on thermal DR.

[0030]FIG. 12 is a graph of the effect of downflow cooking according tothe present invention on extraction-filtrate conductivity.

[0031]FIG. 13 is a graph of the effect of downflow cooking according tothe present invention on oxygen delignification efficiency.

[0032]FIG. 14 is a graph of the effect of downflow cooking according tothe present invention on brownstock yield.

[0033]FIG. 15 is a graph of the effect of downflow cooking according tothe present invention on pulping selectivity.

[0034]FIG. 16 is a graph of the effect of downflow cooking according tothe present invention on cooking uniformity.

[0035]FIG. 17 is a graph of the effect of downflow cooking according tothe present invention on EA profiles.

DETAILED DESCRIPTION OF THE INVENTION

[0036] The present invention may be understood more readily by referenceto the figures, detailed description, and the examples, which follow,where like reference numerals represent like elements throughout, unlessthe context clearly dictates otherwise.

[0037] It is to be understood that this invention is not limited to thespecific methods, conditions and/or parameters described, as specificmethods and/or method conditions and parameters may, of course, vary. Itis also understood that the terminology used herein is used for thepurpose of describing particular embodiments only and is not intended tobe limiting. It must also be noted that, as used in the specificationincluding the appended claims, the singular forms “a,” “an,” and “the”include plural references, unless the context clearly dictatesotherwise.

[0038] Ranges may be expressed herein as from “about” or “approximately”one particular value and/or to “about” or “approximately” anotherparticular value. When such a range is expressed, another embodimentincludes from the one particular value and/or to the other particularvalue. Similarly, when values are expressed as approximations, by use ofthe antecedent “about,” it will be understood that the particular valueforms another embodiment.

[0039] The present invention (based upon functioning commercial systems)demonstrates that the ability to extract from a circumferential digesterscreen section is dramatically enhanced if: (a) the free liquor flow iscon-current (i.e., liquor flows down) to the chip flow at any/all screenlocations, and (b) the free liquor downflow does not exceed 400 (±300)gallons per ton of b.d. wood feed, preferably anywhere in the vessel, asmeasured. The enhanced extraction ability refers herein to (a) abilityto extract more per unit area of screen section before plugging or chipcolumn consolidation occurs, and (b) the ability to extract uniformlyacross the circumference.

[0040] As stated above, the process and configuration according to thisinvention creates free liquor downflow. Also, excessive downflow isavoided. Free liquor downflow cannot be measured directly and can onlybe estimated by calculation. The calculations require that severalassumptions be made and are based upon measured inputs that will havesignificant errors. As a result, calculated values for free liquordownflow will have significant margins of error. For a 1000 ton pulp perday system, errors of 200 gpm (or 100% of targeted values) are notuncommon.

[0041] In order to estimate the potential magnitude of calculationerror, a real-time, overall mass balance must be performed. The sum ofall measured or inferred inflows is subtracted from the sum of allmeasured or inferred outflows.

Mass of Inflows=Mass of wood as wood chips+Mass of wood moisture+Mass ofcondensed steam+Mass of total cooking chemical added+Mass of total washfiltrate added+Mass of seal water leakage into the system,  Equation(1).

Mass of Outflows=Mass of total extraction liquor+Mass of filtrate inoutflowing pulp slurry+Mass of pulp fiber in out flowing pulpslurry+Mass of all vented steam,  Equation (2).

Error in Mass Balance=Mass of Inflows−Mass of Outflows,  Equation (3).

[0042] To a first approximation, it can be assumed that the error incalculated liquor flows will be of the same order of magnitude as theError in Mass Balance. A suitable (initial) control target for each zonein the digester is a free liquor flow of 100 gpm+the absolute value ofthe error. Final target flows will be adjusted based on system response.the Total Liquor Flow, the Chip-Bound Liquor Flow, and the Free LiquorFlow. By definition:

Free Liquor Flow=Total Liquor Flow−Bound Liquor Flow  Equation (4).

[0043] The liquor flows have a vector component and so a convention fordirection is necessary. For example, liquor flowing vertically downwardcan be assigned a positive value in which case upflowing liquor wouldhave a negative value. Also, calculations are based on a discretecontrol volume and so the physical location of the calculated flow mustbe specified: e.g., at the bottom of the control volume.

[0044] To solve Equation (4), the Bound Liquor is estimated and theTotal Liquor flow is determined based on a mass balance around the zonein question. The bound liquor is calculated based upon an assumption ofthe yield, the original basic wood density, and an assumption for theaverage degree of chip volume compression in the zone of interest.

[0045] The hydraulic condition of the digester is determined bycalculating Total, Free, and Bound liquor flows on a zone-by-zone basisusing an iterative calculation. This iterative calculation can beperformed from the “top-down” or from the “bottom-up”. For illustrativepurposes, the bottom-up calculations will be summarized below.

[0046] The bottom most zone of the digester is the blow dilution zone.Here, wash filtrate is added into the zone and pulp slurry leaves thezone.

[0047] By definition, based on the conventions specified above:

Total Liquor flow=Blowline Flow,  Equation (5).

[0048] Furthermore,

Blowline Flow=Total Flow from Preceding Zone+Filtrate Added in BlowDilution Zone  Equation (6).

[0049] From Equation (6), the Total flow from the preceding zone (whichis the wash screen section) can be calculated. The hydraulics withinthis preceding zone are then calculated using a similar method:

Total Flow from Wash Screens=Total flow from preceding Zone+filtrateadded to wash screen zone+cooking chemical added to wash screenzone−extraction from wash screen zone,  Equation (7).

[0050] Thus, Equation (7) can be used to calculate the preceding zonesTotal Liquor Flow.

[0051] Using this iterative approach, the total flow for each zone ofthe digester can be calculated. Once total flows are determined thenEquation (4) is used to calculate the free liquor flow in each zone.

[0052] As a result of uniform and high capacity extraction, veryefficient radial displacement of downflowing liquor can be achieved(i.e., displacement with, for example, a liquor stream introduced at thesame elevation through a central pipe downcomer). This efficientdisplacement can be used at the very bottom of a digester to affectexcellent washing and heat recovery. It has also been found that theenhanced extraction capacity is aided by substantially eliminating anyupflowing free liquor anywhere in the digester vessel, although thisconstraint is not as important as maintaining downflow in the screensections. Most every digester (including the downflow cold blowdigesters from the 1950's) affected an upward component of the freeliquor velocity at one or more extraction screen locations as a resultof their design balance. Furthermore, the vast majority of operatingsystems in the world will have such an upward velocity component.Notable exceptions here are severely overloaded systems with excessivelyhigh free liquor downflows.

[0053] Accordingly, the present invention is directed to a process anddigester design configuration, which take advantage of this observationwhile utilizing other design configurations that are known to enhanceoverall process performance.

[0054] In on embodiment, the process of the present invention preferablymaintains a downflow substantially throughout the entire reactionvessel, particularly at each screen section, and more particularly atthe screen walls. In order to achieve this while avoiding excessivedownflow liquor velocities, multiple extractions are employed. Multipleextraction sites further enhance the overall extraction ability of thesystem. The enhanced extraction ability, and the absence of upflowingliquor both allow for significant reductions in the vessel diameter forany given production rate.

[0055] Multiple extractions dictate the use of multiple WL additions.Preferably, WL additions are made simultaneously with (or downstream of)the individual extractions so as not to have excessive residual chemicalin the extraction liquor flows.

[0056] In order to minimize the well known, negative effects of highlignin concentration at the end of bulk delignification, the process anddigester of this invention is configured to affect an external upflow.This is accomplished by using jump-stage recirculations, whereinextracted liquor from one elevation is reintroduced at a higher (orupstream) elevation.

[0057] While singular jump-stage recirculations have been used elsewhere(e.g., from the wash to the extraction in overloaded commercial systems,or from the secondary to the primary heating circulation), the presentinvention is unique, in one embodiment, in that it uses a plurality ofsuch stages throughout the entire bulk and residual delignificationstages.

[0058] As such, the resultant internal-downflow, external-upflow processmaintains a downflow substantially throughout the entire vessel whileaffecting a process flow scheme that is essentially counter-current (andmulti-staged) throughout the whole of bulk and residual delignification.

[0059] The internal-downflow, external-upflow of the present inventionis analogous to the process configuration of a counter-currentbrownstock or counter-current bleach plant washing trains in that thesolid and liquid phases move con-currently within the unit operationitself, but the process involves a solid/liquid separation wherein theliquid is externally transferred to an upstream stage. The practice ofextracting some portion of counter-flowing liquor has also been appliedin bleach plant systems. The type of a multi-stage, internallycon-current (downflow)/externally counter-current (upflow) system of thepresent invention has never, however, been practiced in or proposed fora continuous digester.

[0060] Multi-stage extraction and/or washing ideally should have anefficient displacement between stages. Another aspect of the presentinvention is that the screen sections, which typically consist of a setof 2 circumferential screen assemblies, are physically separated by atleast 3 linear feet of blank plates. This rest area between screens ismeant to provide a sharp interface between the end of one zone (stage)and the beginning of the next. It is also felt that this relaxationbetween screens will further enhance extraction capacity and extractionuniformity by reversing the effects of chip column compaction at the 1stextraction screen assembly of any set of screens.

[0061] This invention can be practiced in 2, 3, 4 or 5 screen elevationsystems or in either a single or twin vessel system.

[0062] Another aspect of the present invention is that every row ofscreens is preferably divided into no less than 6 individual screenplates, and that liquor is withdrawn from behind each screen plate (outof the reactor) through an individual nozzle followed by individual flowindicator-control instruments. This practice provides maximum diagnosticand control capabilities thus insuring optimal uniformity and capacityfrom each screen assembly or row.

[0063] Another aspect of the present invention is that the process andconfiguration preferably includes a computer software program that usesprocess data from process instrumentation to calculate free liquorvelocity and net direction at every elevation of the reactor. Thesecalculations outlined in detail above, are performed in “real time”(i.e., no less than once every hour) and their results are reported tooperating personnel. Another aspect of this invention is that theassociated software will allow operators to simulate the effect ofprocess operating set point changes on liquor profiles, and will alsorecommend changes required to insure the free-liquor flow requirementsare met.

[0064] The Pulping System

[0065] Referring particularly to FIG. 1, and to provide an example andoverview of a pulping system and process, a two-vessel hydraulic 1200admt/d system operated in an Extended Modified Continuous Cooking(EMCC™) mode, shown generally at 155. The digester comprises a top and abottom, an inlet at the top for receiving cellulosic fibrous materialand an outlet at the bottom for discharging digested pulp.

[0066] In particular, the digester system 155 is fed wood chips fromwood yard area 171. The wood chip stream 201 is fed to the digester feedsystem 301. A chip-liquor slurry 401 is fed to the top of digestervessel 70 having a chip-liquor separator 50. A liquor return stream 601is fed back to the digester feed system 301.

[0067] The digester system 155 has a primary heating circulation system90 and a secondary heating circulation system 100, both with an indirectliquor heater. A circumferential screen assembly for cook liquorwithdrawal is located at 80 a-c along the length of vessel 70. Liquor iswithdrawn from locations 80 a, 80 b and 80 c and circulated to heatingsystems 90 and 100, and re-circulated back to the vessel 70 through aplurality of jump stages, wherein the “upflow” of liquor is accomplishedexternally.

[0068] The circumferential screen assembly 80 b for cook liquorwithdrawal is approximately mid-vessel and extracts a spent cookingliquor stream 110 and directs that stream to a liquor evaporation area120. After evaporation, concentrated, spent (or “Black”) liquor 130 isdirected to chemical recovery and preparation areas 140. A “white”liquor stream 150 (cooking chemical) is fed to digester feed system 301for forming a chip-liquor slurry for introducing into the top of vessel70.

[0069] The blow dilution zone of vessel 70 is shown generally at 160.Blow dilution zone 160, located around the bottom area of digestervessel 70, has a side dilution header/nozzle assembly 170 and a bottomhead dilution header/nozzle assembly 180. At the very bottom of thevessel 70, a blowline 190 containing a pulp/water slurry stream directsthe slurry to a “brownstock” washing and screening area 200. Wash water210 is added to the brownstock washing and screening area 200. From area200, filtrate 220 (also called cold blow filtrate or wash filtrate) isdirected from the wash to the digester's blow dilution zone 160. Washedbrown stock pulp/water slurry 230 is directed from the washing andscreening area 200 to bleaching and/or drying and/or paper making areas240.

[0070] Cooking chemical is added at both the wash and MCC™ circulations.Simultaneous counter-current cooking and washing take place in the zonesbetween the extraction and wash screens. Cooking total retention time isbetween 4.5 and 5.5 hours, with approximately 3.5 to 4 hours retentionin the counter current zones.

[0071] Not shown in FIG. 1, but following digester 15 typically is a2-stage atmospheric diffuser, pressurized screen room and vacuum washerdecker. A 2-stage medium consistency oxygen delignification systemfollowed by atmospheric diffuser and vacuum washer complete thebrownstock fiberline system. The fiberline system produces severalgrades of softwood market pulp from a variety of coastal and interiorwood furnishes.

[0072]FIG. 2 shows a simplified schematic view of a continuous digesterand process of this invention including a digester reactor 2 with three(3) screen sections (1 a, 2 a and 3 a from the top-down). The digesterconfiguration and process can be extended to include systems with 4, 5or more screen sections, if desired.

[0073] Wash water or filtrate 6 is added to the center of the digester 7in addition to being added in the blow dilution zone 8. This wash water6 is added slightly above screen 3 a elevation through a central pipeassembly (not shown).

[0074] Liquor withdrawn from the screen 3 a is pumped through jump-stagerecirculation line 9 to the central pipe discharge at or slightly abovethe screen 2 a elevation. Likewise, liquor withdrawn from the screen 2 ais pumped through jump-stage recirculation line 10 to the central pipedischarge at or slightly above the screen 1 a elevation.

[0075] For the embodiment shown, spent liquor is extracted from thesystem 11 and to recovery operations through the screen 1 a.

[0076] For existing systems, screens 1 a and 2 a typically have two rowsof screen plate sections. FIG. 3 illustrates an extension of the basicconcept described herein wherein an additional jump-stage recirculationflow 12 is taken from screen 1 a and sent to the feed system 15 whereincoming wood chips and feed liquor 16 are introduced.

[0077] A preferred method for extending the process as such would be toselectively extract liquor to recovery from the top row 13 of screen 1 aand recirculate or jump from the bottom row 14. To achieve thisadditional recirculation from screen 1 a to the feed, the recirculatedliquor would have to be cooled in a cooling unit 17 such as a heatexchanger or a flash cooling device.

[0078] The process and digester configuration described and shown byFIG. 3 illustrates that multiple jump recirculation flows are used toaffect an external upflow, internal downflow throughout the entiresystem.

[0079] In application, there will be circulation systems in place ateach of the screen elevations. These circulations are used to introduceheat and cooking chemical to the system at the given elevation. The jumpstages will be incorporated into the circulations as shown schematicallyin FIG. 4, where circumferential digester screens are shown at 24.

[0080] Note that it in the preferred embodiment of the invention, thejump-stage recirculation lines 18 are taken downstream of thecirculation pump 19, but upstream of the WL and/or filtrate additionpoint 20 and upstream of the circulation's heater 21. Furthermore, theentry point 22 for any given re-circulation (i.e., into the precedingcirculation system) would have to be downstream of any extraction 23 orre-circulation 18 flow drawn from the receiving circulation.

[0081] The external upflow process can be operated with multipleextractions. FIG. 5 is identical to FIG. 2 except that three extractionlines are utilized (i.e., spent liquor is extracted off of screensections 2 a and 3 a in addition to screen section 1 a). Extractionflows off of screens 2 a and 3 a are denoted as 26 and 27, respectively.

[0082] Extraction flows 26 and 27 are not desirable in that they willindirectly result in decreased energy and washing efficiencies. Thegreater the flow of either 26 or 27, the greater the loss in bothwashing and energy efficiency. These flows would only be utilized ifextraction flow 11 were capacity limited (i.e., if the screen extractioncapacity was not large enough to induce full uptake of available washfiltrate). In general, extraction flow 11 is to be maximized andextraction flows 26 and 27 would be minimized (i.e., set to the minimalflow required to achieve an overall extraction flow which results inuptake of all available wash filtrate). In general, use of extractionflow from 26 would result in smaller energy and washing penalties thanuse of extraction flow from 27.

EXAMPLES

[0083] The following examples and experimental results are included toprovide those of ordinary skill in the art with a complete disclosureand description of particular manners in which the present invention canbe practiced and evaluated, and are intended to be purely exemplary ofthe invention and are not intended to limit the scope of what theinventors regard as their invention. Efforts have been made to ensureaccuracy with respect to numbers (e.g., amounts, temperature, etc.);however, some errors and deviations may have occurred. Unless indicatedotherwise, temperature is in ° C. or is at ambient temperature, andpressure is at or near atmospheric.

[0084] An extended digester trial was performed, wherein the digesterprocess configuration was changed from upflow to downflow modifiedcooking. The primary objective of the trial was to quantify the effectof downflow cooking on overall mill operations and then use thisinformation to perform a cost-benefit-risk analysis required to convertto permanent downflow operations. The secondary objectives were toidentify potential operating constraints and to obtain operating dataand results for use in final process design.

[0085] Operating data, technical data from pulp and liquor testing, andprocess modeling were used to characterize process performance before,during, and after the trial. The results presented at least show thatdownflow operations gave: (1) improved K# and level control; (2) higher,more stable blowline consistency; (3) improved digester washing andenergy recovery efficiencies; (4) improved brownstock diffuser washing;and (5) improved oxygen delignification efficiency. The validatedprocess model predicts no effect on viscosity or yield, but predictsfurther improvements in washing efficiency for a permanentconfiguration. Independent sources of cost savings were identified andquantified, and these results were used to predict the effectiveness forthe permanent conversion project.

[0086] In particular, a two-vessel hydraulic digester that cooks aspenhardwood and white spruce softwood in campaigns was used. Presentproduction rates for hardwood are substantially over design, 1680⁺admt/d digester production verses the original design of 1250 admt/d.

[0087] Increased production rates have resulted in decreased overallbrownstock washing performance. In particular, as rates increased, thesustainable upflow in the digester's counter-current wash zone hasdecreased. This has resulted in less digester washing capacity and lowerwashing efficiency. The efficiency of other washers in the brownstocksystem has also decreased.

[0088] To remedy the loss of brownstock washing, the mill converteddigester operations to Lo-Solids® cooking and has implemented advanceddigester control strategies. Both measures have provided improvedwashing performance and overall digester control is good to excellent.Nevertheless, limitations in overall brownstock washing have reached thepoint to where they are limiting the extent of delignification and theselectivity of the mill's medium consistency oxygen delignificationsystem. Future plans are to further increase production, and this willmake the negative effects of brownstock washing limitations even morepronounced. Finally, even with the present mode of Lo-Solids cooking inplace, several digester limitations exist which will prohibit furtherproduction rate increases; namely, extraction flow limitations, flashsystem limitations, and filtrate/blowline cooling limitations.

[0089] Previous results showed that all of the benefits of modifiedcooking (as well as excellent washing performance) could be obtained fordigesters with little or no upflow by utilizing a downflow Lo-Solidscooking configuration. More recent results showed that conversion to adownflow mode of cooking specifically addressed the limitations of theknown systems and resulted in improved overall digester operations andperformance. The systems are typically severely overloaded two-vesselhydraulic digesters with similar shell and screen configurations.

[0090]FIG. 6 illustrates the presently known “cross-flow” Lo-Solidsconfiguration and FIG. 7 shows two upgrades that permit permanentoperation in a downflow mode.

[0091] The basic purpose of the trial was to develop a more rigorouscost-risk-benefit analysis. More specifically, the trial objectives wereto:

[0092] Identify and quantitatively predict effect of downflow operationson digester stability;

[0093] Identify and quantitatively predict effect on digester yield,washing efficiency, and energy efficiency;

[0094] Identify and quantitatively predict effect on other mill unitoperations (brownstock washing, O₂ delignification, evaporators, etc.);

[0095] Identify operating constraints that may influence final processand equipment design; and

[0096] Obtain estimates for capital effectiveness.

[0097] Trial Methodology

[0098]FIG. 8 compares the base case and trial configurations. The trialconfiguration is a Lo-Solids-type process, wherein spent cooking liquoris extracted during the bulk delignification stage and is replaced withpre-heated white liquor and filtrate.

[0099] Compared to base case operations, the trial mode in accordancewith the present invention involved operating the bottom two cook zonesof the digester in a downflow mode (i.e., versus upflow). In thisparticular case, (the trial mode configuration of the presentinvention), the top most set of screens (i.e., the “Extraction” screens)were not used during the trial and were taken off line. Instead,extractions were performed at the 2^(nd) and 3^(rd) set of digesterscreens from the top down (i.e., the “Modified” and “Wash” screens,respectively). Primary heating shifted from the bottom circulation andheaters (HR) to the modified circulation and heater (HR). White liquor(WL) addition to the wash circulation was turned off and replaced withfiltrate addition at this location. The wash circulation heater was notused during the trial.

[0100] The major differences between the trial configuration, the basecase, and the proposed permanent configurations are seen by comparingFIGS. 6 and 7. Table 1 summarizes the relevant differences. The trialconfiguration resulted in one less cooking circulation (i.e., one lesswhite liquor addition point with heating) than either the base case orthe proposed permanent configurations. Also, the trial mode provided foronly 2 extraction sites versus the 3 extraction sites in the proposedpermanent configurations. As such, the trial mode was operated with only2 independent cook zones versus the 3 independent cook zones availablefor either the base case or the proposed permanent downflowconfigurations.

[0101] Table 1: Comparison of Base Case, Proposed Permanent Downflow,and Trial Configurations. “WL” Refers to White Liquor. Number of WLNumber of Number of addition points Extraction Sites Independent todigester from digester Cook Zones Base Case: MCC 2 2 3 Cross-flowProposed Down- 2 3 3 flow Configurations Trial Down-flow 1 2 2Configuration

[0102] The practical effects of having one less cook zone for the trialmode than for either the base case or proposed permanent downflowconfigurations are: the peak cooking temperature would have to beraised, the residual alkali profile would have higher maximum values andlarger gradients, and overall digester washing would be compromised.These conditions would result in decreased cooking uniformity,viscosity, and yield—all other things being constant.

[0103] Thus, it was recognized in advance that the trial downflowconfiguration would give results that were less favorable than thepermanent downflow configuration(s) and, in some cases, even lessfavorable than the present upflow base case. Nevertheless, the trialstill met its objectives and provided valuable insights into theanticipated performance of the proposed modifications.

[0104] In order to predict operating results for the permanentmodification(s), the base case and trial operating results were used tovalidate a predictive process model of the digester. The validated modelwas then used to predict and compare anticipated results for variouspermanent configurations. Specifically, the trial program consisted ofthe following elements:

[0105] (1) All relevant and available process data from before, duringand after the trial were downloaded from the mill's DCS system; and

[0106] (2) Pulp and liquor samples from before, during, and after thetrial were collected and tested.

[0107] Information from items (1) and (2) were used to characterize thebase case and trial modes as well as validate the predictive digestermodel. The validated model was used to predict the outcome for proposedpermanent downflow configurations.

[0108] Results

[0109] Two trials were performed. The first trial consisted of a 72-hourcampaign. Conversion to downflow provided:

[0110] Improvements in column movement stability, blowline consistency,blowline consistency control, and digester level control;

[0111] An increase in maximum sustainable extraction capacity—from 125L/s to 165 L/s;

[0112] A decrease in filtrate cooler duty wherein cooling water valveposition went from 100% (out of control) to less than 10% in control;and

[0113] Improved blowline temperature control.

[0114] A second trial lasted for 8 days (inclusive of transition timesto go on and off downflow mode). It should be noted that during bothtrials the ability to optimize process conditions was severely limitedby (a) the absence of a second white liquor and heating circulationwithin the digester, (b) modified-circulation heater capacitylimitations, and (c) limitations to wash extraction flow imposed by thesize of the line and valve. The heater limitations were particularlysevere in that they indirectly resulted in limitations to digesterextraction capacity and washing efficiency.

[0115] Effect of Downflow Operations on Process Stability and Control

[0116] During the trial, overall digester stability and control wereexceptional. In many respects, the noted improvements in stability aredifficult to quantify. For example, the mill had an excellent processcontrol system in place and overall process control for the base casewas good to excellent. One of the general observations from operatingpersonnel was that the system responded more predictably and morereadily to control action and that less control action was requiredwhile on downflow mode. While this is clearly a positive effect, itcannot be easily measured. Nevertheless, quantitative results showingimproved control were obtained.

[0117] The long-term blowline K# variance for base case operations was5.4%. This value is from pooled, filtered data over several months andrepresents excellent K# control—particularly for an overloaded,high-capacity two-vessel digester. The trial K# variance was 3.6%, or33% lower than the long-term average. This is a very surprising result,particularly since: (a) throughout the trial process conditions werebeing changed in an effort to optimize the new cooking configuration,(b) the trial configuration had significant limitations on primaryheating capacity and this limited K# control action, and (c) the samplespace for data was substantially lower for the trial than for the longterm result.

[0118] The improved K# control is felt to be the direct result ofimprovements in both digester level control and digester dischargecontrol. Both level and discharge control, in turn, are felt to bedetermined by column movement stability. Table 2 summarizes results forimproved digester level and discharge control. Variability in blowlineconsistency is used here as an indication of discharge control. TABLE 2Improvements in Digester Level Control and Digester Discharge Stability.Outlet Digester Device Level % Variance dP % Variance Long term avg. for43.1 34 332 11.8 base case Trial avg. for down-flow 43.0 29 420 4.2 %Diff., trial vs. base 0% −15% 27% −27% case

[0119] The results in Table 2 show that the variance in level controland outlet pressure drop decreased by 15 and 27%, respectively, whencomparing downflow trial results to long-term averages for base caseoperations.

[0120] Another important process control issue involves the effect thatdownflow operations had on filtrate-cooler duty. The filtrate cooler iscapacity limited when a river or water body temperature is higher, andso normal operations regularly require the cooling water valve openingto be 100%. In other words, the filtrate temperature cannot be decreasedenough to sufficiently cool the blowline and the blowline temperature isconsequently out of control. For base case operations, the blowlinetemperature will often exceed the upper limit permissible for thedownstream atmospheric diffuser. When this happens, the diffuser isby-passed and the entire brownstock washing system is severely upset. Ondownflow mode, the filtrate-cooler duty was decreased dramaticallyallowing blowline temperature to be brought into control. FIGS. 9 and 10illustrate the stepwise changes in blow discharge and in filtrate-coolerduty as a result of transition to and from downflow cooking.

[0121] Effect of Downflow Operations on Process Performance: HeatRecovery and Washing Efficiencies

[0122] The dramatic decrease on filtrate-cooler duty described above isdue to enhanced internal heat recovery for the downflow mode. The radialdisplacement of hot, down flowing fiber and liquor with unheatedfiltrate at the wash screen elevation is extremely efficient atdisplacing heat prior to blowing. Since this heat flows through the washextraction line to the flash tanks, the filtrate need not be cooledexcessively in order to lower the blow discharge temperature, and so thefiltrate-cooler duty is decreased.

[0123] Besides improving blow temperature control, the other practicaleffects of this enhanced internal heat recovery are to decrease theamount of warm water generated in the filtrate-cooler and to increasethe amount of energy present in the extraction liquor. Since extractionliquor is sent to the flash tanks, a portion of this energy is recoveredin the form of low-pressure steam that is subsequently used forpre-heating the incoming chip stream. Thus, warm water generation isdisplaced by low-pressure steam generation.

[0124] A useful and practical method for quantifying internal heatrecovery efficiency is by calculating the thermal displacement ratio(DR_(T)) for the digester's blow dilution zone. As is the case forwashing displacement ratios, an ideal displacement of the component ofinterest (in this case heat) by the incoming displacement medium (inthis case wash filtrate) corresponds to a DR value of 1.00. The lowerthe displacement ratio, the lower the efficiency. For long-term, basecase operations the average DR_(T) was determined to be 0.82 with avariance of 4.8%. For the downflow trial according to the presentinvention, the average DR_(T) was determined to be 0.94 with a varianceof 2.1%. The difference between upflow and downflow efficiencies was0.12 points, or 14.6%, with downflow mode having higher efficiency. Thisis an extremely large difference for this parameter. The observed ratioof 0.94 for down flow operations is extraordinarily high compared totypical results for upflow systems but typical of the author'sobservations for downflow systems. The difference between upflow anddownflow variability in DR_(T) was 2.7 points or 56%, with downflow modehaving lower variability. This decreased variability is anotherconfirmation of improved digester discharge stability for the downflowmode.

[0125]FIG. 11 shows the stepwise change in thermal DR as a result oftransition to and from downflow cooking.

[0126] In general, the thermal and wash displacement ratios are relatedto one another because both will depend on the filtrate-to-chipcontacting efficiency. Since thermal DR was shown to increase as aresult of downflow operations, it was expected that the digester'swashing efficiency would also increase. Furthermore, digester extractioncapacity increased from 125 Us to an average of 142 L/s (and peaked at165 L/s) while on the trial mode according to this invention. Thedifference of 17 L/s as compared to upflow operations correspondsdirectly to an increase in washer filtrate uptake—or increased digesterdilution factor. The increased dilution factor is another reason why thedigester washing efficiency increases for the downflow according to thisinvention.

[0127] Extensive sampling of blowline squeezate and brownstock filtrateswas performed during and long after the trial. For the data reportedhere, over 19 sets of samples were collected.

[0128] Samples were tested for sodium and COD in order to directlymeasure washing efficiencies. Table 3 summarizes the results. TABLE 3Summary of COD profiling Upflow Downflow % Avg. % Var. Avg. % Var. Diff.Blowline 109939 10 84723 13 −23 2 Stage Atm. Diff. 38757 19 30200 12 −223A/3B Filtrate 32640 20 30843 12 −5 #5 Squeezate 1649 15 1577 15 −4

[0129] Converting from upflow to downflow operations resulted in adecrease in blowline solids of between 20 and 25%. For example, theaverage blowline COD concentration for upflow operations wasapproximately 110,000 mg/L with a variance of 10%, whereas the averageconcentration for the downflow trial was approximately 85,000 mg/L witha variance of 12%. The larger variance is typical of results for directtesting of filtrate solids.

[0130] Measured values for global digester wash DR were 0.82 (8.5%variance) for upflow operations and 0.86 (8.8% variance) for downflowoperations. The difference of 0.04 points, or 5%, is a large value forthis parameter. The variance, or imprecision of data, is compounded whenconverting from direct concentration measurements to the efficiencyparameter. The large variability in DR is typical of wash efficiencystudies and speaks to the practical challenges of performing directmeasurement washing studies.

[0131] During the trial, the total increase in filtrate uptake waslimited to 40 L/s, and averaged 17 L/s, due to limitations in themodified circulation heater and in the flow capacity for the washextraction line. Neither of these limitations would exist on a permanentre-configuration. In the absence of these limitations, it is anticipatedthat the total digester extraction would increase to the currentevaporator capacity of 160 L/s and that the corresponding increase infiltrate uptake would be 35 L/s. For a permanent configuration,evaporator capacity would be the ultimate limitation to digesterextraction capacity and filtrate uptake. Once again, any additionalincrease in filtrate uptake will result in further increases in digesterwashing efficiency.

[0132] Effect of Downflow Operations on Other Areas

[0133] As shown in Table 2, downflow cooking resulted in an increase inblow consistency of approximately 25% and a simultaneous decrease inconsistency variability. It is commonly known that increased stabilityand consistency to the inlet of a diffusion washer results in improvedoperations and efficiency. Table 3 shows that diffuser extractionfiltrate also decreased by approximately 20%.

[0134] Testing around the diffuser showed that the unit's wash DRincreased from 0.89 (variance of 5.6%) for upflow to greater than 0.96(variance of 9.9%) for downflow. The difference of 0.07 points, orapproximately 8%, represents a large increase for this parameter. Aswith the digester DR calculations, it is seen here that the variance ofdata increases substantially when converting from direct concentrationmeasurements to the DR efficiency parameter. For both the digester anddiffuser, differences between upflow and downflow filtrateconcentrations are statistically significant at a relatively highconfidence interval whereas differences in wash DR's are not.

[0135] Thus, both digester and diffuser wash efficiencies increased byrelatively large amounts as a result of downflow operations. Thisconclusion is supported by testing results, but to a limited degree ofstatistical confidence. This conclusion is also supported by thefollowing observations: (a) thermal DR's increased substantially andsignificantly, and there is a strong relationship between thermal andwash DR's since both are a function of chip-to-filtrate contactingefficiency; (b) digester global dilution factor increased substantiallyand it is known that washing efficiency increases with increaseddilution factor, and (c) diffuser inlet consistency increased and bothinlet consistency and inlet temperature control improved—theseconditions are expected to improve diffuser efficiency.

[0136] Perhaps the biggest potential effect on downstream washing,however, is related to improved blow temperature control and theanticipated elimination of severe upsets due to diffuser by-pass on highblow temperatures. FIG. 12 shows the diffuser extraction filtrateconductivity during the trial. Filtrate conductivity is only an indirectmeasure of washing effectiveness since it will also depend on wash waterpurity, the chemical charge to the oxygen reactor, and the oxygenreactor's extent of delignification. Nevertheless, it is a goodqualitative indicator of system response to overall conditions.

[0137] As shown in FIG. 12, the trial commenced July 7^(th) Between July7^(th) and 8^(th), the digester was undergoing transition from upflow todownflow and the extra wash water make-up flow was being graduallyeliminated so as to bring the brownstock washing net dilution factorback down to normal levels. By the end of July 8^(th) brownstockconditions were at target set points. Between July 9^(th) and July11^(th) both the digester and diffuser ran stable. Filtrate conductivitysteadily decreased from a peak value of 36 to slightly below 28 andappeared to be trending further downward. Normal values for extendedstable operations are between 32 and 34. The minimum value of 28observed during the trial is exceptionally low.

[0138] The diffuser was diverted for less than 3 hours due to amechanical (switch) failure. Once brought back on-line, the conductivitywas found to have increased from 28 to 34. It took fully another 24hours of stable operation to return to approximately 31, i.e., 24 hoursto reverse ½ of the effect of a 3-hour upset.

[0139] The second to last day of the trial, a process change was made tothe digester in an effort to further optimize extraction rates.Unfortunately, this change proved to be a misstep that caused a systemhydraulic upset. Blowline temperatures increased and the diffuser wasby-passed for approximately 2.5 hours. As a result, diffuserconductivity increase from 31 to 38 and it took fully 2 days ofnormalized operation to return to the 31 level.

[0140] The results in FIG. 12 shows that extended, stable operations indownflow mode result in low filtrate conductivity values. The relativeeffects on diffuser filtrate conductivity are not as large as onblowline and diffuser filtrate COD concentrations. This is likely due toother variables influencing filtrate conductivity values (e.g., chemicalcharge in oxygen reactor and wash water purity). These results alsoillustrate the dramatic impact of relatively short system upsets.Typical response time to a 2 to 3 hour upset is between 2 and 3 days.Even after 3 days of stable operations no apparent minimum was observed.Hot blowing and simultaneous diffuser by-pass is substantially worsethan diffuser by-pass alone: this is because hot blowing corresponds toloss of digester heat recovery and washing efficiencies.

[0141] While the hot blow excursion in the middle of the trial wasunfortunate, downflow operations will significantly reduce the frequencyof such hot blows due to (a) enhanced discharge stability, (b) enhancedsystem response to control action, and (c) greater control range onfiltrate-cooler discharge temperature. For example, during the excursionof July 12^(th), the filtrate-cooler discharge temperature did notchange quickly enough to avoid high blowline temperatures—even thoughthere was ample cooler capacity for this purpose. The feedbacktemperature control loop was not properly tuned for the new, morerapidly responding conditions. It is felt that, with more run time andappropriate adjustments to control loop tuning, the operators andcontrol system will be able to avoid future hot blows under similarcircumstances.

[0142]FIG. 13 shows the response of the oxygen delignification systemduring the trial. The long-term % delignification average for upflowoperations is approximately 27%. Achieving 30⁺% delignification atnormal production rates is unusual.

[0143] During the trial, the extent of delignification increasedsteadily up to the point where the atmospheric diffuser was taken offline. The improved performance is related to improved brownstockwashing. The system response to improved washing confirms that theoxygen delignification system is carry-over limited: i.e., that upstreamwashing limits the performance of this delignification stage. The oxygensystem has a 2 to 3 day response time, which is consistent with theresponse (or lag) time exhibited by the brownstock filtrate cycle. Aswith the brownstock filtrate cycle, it appears as though the maximumpotential system response was never achieved during the course of thetrial: i.e., it appears as though filtrate conductivity did not achieveits apparent minimum and so % delignification did not achieve itsapparent maximum. During the trial, the peak value for extent ofdelignification was approximately 15% higher than the long-term averagefor upflow operations. Long-term downflow operations will result in morethan a 15% increase in the average extent of delignification.

[0144] It is interesting to fully consider the inter-active effectsbetween brownstock washing efficiency and extent of oxygendelignification. Since delignification is washing limited, anyimprovement in digester and/or diffuser washing will result in increased% delignification within the oxygen reactor. This increase in extent ofreaction results in the formation of more dissolved organic solidswithin the reactor—most likely up to the point where the system becomeslimited again. The auto-limited generation of additional solids resultsin increased COD concentration for post-oxygen washer filtrate. Thisfiltrate, in turn, is re-circulated backwards to the pre-oxygen washer.Thus for a carry-over limited system such as this, it is expected thatimproved upstream washing (i.e., decreased carry-over from thedigester/diffuser) will result in increased delignification up to thepoint where limiting concentrations are achieved again. Further, it isexpected that filtrate concentrations around the oxygen delignificationstage will remain relatively constant and at carryover concentrationsthat correspond to the limiting threshold for further reaction at thegiven level for % delignification. The results presented in Table 3suggests that this is precisely what happened during the trial. Digestersqueezate and diffuser filtrate concentrations decreased by more than20% whereas pre- and post-oxygen filtrate concentrations decreased byless than 5% as a result of the trial. The small differences in pre- andpost-oxygen filtrate concentrations have an extremely low level ofstatistical significance.

[0145] Improved digester and diffuser washing result primarily inincreased extent of oxygen delignification. Little or no decreases inCOD carry-over into the bleach plant where observed or are expected.Lower COD concentrations in the pre-oxygen area of the brownstockwashing system may have secondary benefits: e.g., less defoamer usage.Another potential secondary benefit is related to the fact it is oftennecessary to add make-up wash water to the brownstock washing system inorder to minimize COD carry-over and post oxygen K# upsets. Loweraverage COD concentrations for the oxygen feed stock due to improvedDigester and Diffuser washing and less Diffuser by-passes will reducethe need for periodic wash-water make-up. The long-term effect of thiswould be to increase average % solids to the evaporators, and todecrease variability in % solids.

[0146] The decrease in filtrate-cooler duty has a significant effect onthe mill's warm/hot water balance. The net effect is to decrease thegeneration of warm water and the mill's overall use of low-pressuresteam. Modifications to the digester heat recovery will allow thisdecrease in low-pressure steam demand to displace medium pressure steam.Thus, the proposed modifications according to this invention will resultin decreased overall steam usage. The filtrate cooler uses fresh water.Decreased filtrate-cooler duty therefore results in decreased freshwater uptake and decreased water treatment costs.

[0147] Effect of Downflow Operations on Pulp Yield

[0148] As described above, the trial configuration resulted in processconditions that were expected to have a negative effect on yield andselectivity. Yield audits were performed in order to assess the shortand long term impacts of converting to downflow cooking.

[0149] Results from the audits produced the following calibration curvefor the aspen hardwood's brownstock yield:

Y=3.75*(log V)/C2+42

[0150] Where Y is brownstock yield (mass of brown pulp per unit mass ofbone dry wood feed), V is the brownstock viscosity, and C is the massfraction of cellulose in pulp. Measured yield values are felt to beprecise to within +/−0.25 yield points. FIG. 14 compares measured yieldresults for upflow and trial downflow operations.

[0151] Based on the data shown in FIG. 14, yield at any given Kappanumber decreased by approximately 0.1 points when comparing long-termupflow to trial downflow operations. While this effect is smaller thanthe limits of precision for the test, it is felt to be a good indicationof the trend and magnitude of actual results.

[0152]FIGS. 15 and 16 show the results for cooking selectivity andcooking uniformity. Brownstock viscosity dropped from 76 to 72 cp duringthe trial. No effect on bleached viscosity was observed. Reject contentappeared to increase marginally, but levels in both cases are extremelylow and analysis is complicated by the inherent imprecision in this typeof testing. It is concluded that yield and selectivity were similar, orperhaps slightly lower, for the trial as compared to typical upflowoperations.

[0153] Validated Process Model Predictions

[0154] Testing data from the yield audits and liquor solids profiling,as well as process data from the mill's DCS were used to validate apredictive, steady state process model of the digester. This processmodel predicts steady-state conditions for alkali profiles, dissolvedsolids profiles, temperature requirements, and pulping kinetics. Theseresults are used to infer effects of a permanent downflow process onpulping yield, selectivity, and washing efficiency.

[0155] As an example, FIG. 17 compares the effective alkali (EA)profiles for upflow conditions, trial downflow conditions, and permanentdownflow conditions. The upflow and trial-downflow profiles werevalidated by direct testing and measurement, whereas the permanentdownflow profile is strictly a model prediction. Note the increase inalkali concentration for trial conditions throughout the digester. Thiseffect, again, is due to the elimination of a cook zone. Permanentdownflow configurations eliminate this effect and, in fact, are expectedto result in an alkali profile that is marginally better (i.e.,“flatter”) than base case upflow conditions.

[0156] The model predicts that both the selectivity (i.e., viscosity ata given Kappa number) and yield are effectively identical for long-termupflow and permanent downflow configurations. The model also predictsthat permanent downflow will result in further improvements in digesterwashing efficiency. While trial conditions resulted in a 25% reductionin blowline solids concentration, the permanent configuration isexpected to affect as much as a 35% reduction in this concentration.

[0157] Modification Results

[0158] Based on the results of this trial, the following beneficialresults have been identified:

[0159] Displacement of other capital projects (upgrade of cold blowcooler and brownstock washing).

[0160] Lower bleach costs from better O₂ delignification.

[0161] Elimination of diffuser bypasses (hot blowing), leading toevaporator off-loading.

[0162] Low and medium pressure steam savings.

[0163] Lower bleach costs from better K# control.

[0164] Fresh water treatment savings due to lower filtrate-cooler duty.

[0165] Less white liquor losses through digester discharge and filtrateby-pass.

[0166] Based on these results, the trial successfully met itsobjectives: quantitative information has been obtained for the impact ofdownflow operations on overall digester performance.

[0167] The impact of downflow operations on digester stability,responsiveness, and runnablility at the mill are now known. The extendedtrial showed that downflow resulted in:

[0168] Improved K# and level control.

[0169] Higher, more stable blowline consistency.

[0170] Improved digester washing and energy recovery efficiencies.

[0171] Improved atmospheric diffuser washing.

[0172] Improved oxygen delignification efficiency.

[0173] The output from a validated digester model predicts that therewill be no effect on viscosity or yield, but further improvements stillin washing for a permanent configuration.

[0174] This invention has been clearly described in detail, withparticular reference to certain preferred embodiments, in order toenable the reader to practice the invention without undueexperimentation. Theories of operation have been offered to betterenable the reader to understand the invention, but such theories do notlimit the scope of the invention. In addition, a person having ordinaryskill in the art will readily recognize that many of the previouscomponents and parameters may be varied or modified to a reasonableextent without departing from the scope and spirit of the invention.

[0175] Furthermore, titles, headings, examples or the like are providedto enhance the reader's comprehension of this document, and should notbe read as limiting the scope of the present invention. Accordingly, theinvention is defined by the following claims, and reasonable extensionsand equivalents thereof.

What is claimed is:
 1. A digester for continuously cooking and digestingcellulosic material and producing cellulosic pulp comprising: aplurality of jump-stage recirculations located along the length of acontinuous digester vessel, wherein each of the jump-stagerecirculations moves extracted liquor from a first elevation of thedigester vessel and re-introduces the extracted liquor into the vesselat a second higher elevation to maintain an internal liquor downflowsubstantially throughout the entire digester vessel.
 2. The digester ofclaim 1, wherein the internal liquor downflow does not exceed 400gallons per ton of b.d. wood feed substantially throughout the entiredigester vessel.
 3. The digester of claim 1 further comprising aplurality of liquor extraction and addition points along the length ofthe digester.
 4. The digester of claim 3 further comprising aflow-indicator-control system associated with each jump-stagerecirculation and liquor extraction and addition points.
 5. The digesterof claim 1, wherein the digester vessel comprises a plurality ofextraction screens along the length of the vessel, each extractionscreen having a jump-stage recirculation.
 6. A digester for continuouslycooking and digesting cellulosic material and producing cellulosic pulpcomprising: a jump-stage recirculation located along the length of acontinuous digester vessel, wherein the jump-stage recirculation movesextracted liquor from a first elevation of the digester vessel andre-introduces the liquor into the vessel at a second higher elevation tomaintain an internal liquor downflow substantially throughout the entirevessel that does not exceed 400 gallons per ton of b.d. wood feed. 7.The digester of claim 6 further comprising a plurality of liquorextraction and addition points along the length of the digester.
 8. Thedigester of claim 7 further comprising a flow-indicator-control systemassociated with the jump-stage recirculation and liquor extraction andaddition points.
 9. A process for continuously cooking and digestingcellulosic material and producing cellulosic pulp in a continuousdigester comprising: (a) maintaining an internal, con-current liquordownflow substantially throughout a continuous digester vessel, whereinthe internal liquor downflow does not exceed 400 gallons per ton of b.d.wood feed.
 10. The process of claim 9, wherein the continuous digestervessel comprises a plurality of jump-stage recirculations located alongthe length of the vessel, and each of the jump-stage recirculationsmoves extracted liquor from a first elevation of the digester vessel andre-introduces the extracted liquor into the vessel at a second higherelevation to maintain the internal liquor downflow substantiallythroughout the entire digester vessel.
 11. The process of claim 9,wherein the continuous digester vessel comprises a plurality of liquorextraction and addition points along the length of the digester.
 12. Theprocess of claim 11, wherein the continuous digester vessel furthercomprises a flow-indicator-control system associated with eachjump-stage recirculation and liquor extraction and addition points. 13.The digester of claim 9, wherein the digester vessel comprises aplurality of extraction screens along the length of the vessel, eachextraction screen having a jump-stage recirculation.